Usually, packaging film for use in sealed packages of food products, drugs and so on are composed of double- or more multilayers including at least a heat-sealable layer and an outer coverage layer. The heat-sealable layer has a role for providing a tight sealing of the package and is made of, for example, polyethylene (PE) or polypropylene (PP). The outer coverage layer should have a heat resistance for standing against the heat upon the heat sealing and a shock resistance required for protecting the packaging (film) and the contents of the package against physical attack from outside and is in general made of, for example, polyester, polyamide (Ny), polypropylene (PP) or cellophane, solely or in combination (composite) of two or more of them.
For the outer layer of the packaging film, a film of polyester, polyamide or polypropylene is employed, which is usually biaxially stretched for further increasing the heat resistance and shock resistance.
In using a cellophane film, so-called "moisture-proof cellophane" in which one or both surfaces are treated by a moistening-protective coating by, for example, vinylidene chloride thin film, is often chosen, since ordinary cellophane has an excessive hygroscopic nature and its material properties will considerably change when moistened.
It is required for a material for the outer layer of a packaging sheet to have, in addition to the superior heat resistance and shock resistance mentioned above, a moisture-barrier property on storage and transport thereof so as to block up intrusion of external moisture into the package to prevent any moistening of the article to be packaged. An "easy hand-tearability", namely, a property for opening the package by tearing the packaging sheet easily by hand for taking out the packaged article is also an important feature required for a packaging film.
In conventional multilayer packaging films, however, none has been found to have better resistances to heat and shock together with a moisture-barriering property and an easy hand-tearability.
Thus, for the laminated films of the stand of technique in which the outer layer is constituted of cellophane, there may be enumerated combinations of, for example, cellophane/PE, moistureproof cellophane/PE, moistureproof cellophane/paper/PE and moistureproof cellophane/paper/aluminum foil/PE. Such a conventional cellophane-base laminated film has, in general, an easy hand-tearability due to the intrinsic nature of cellphane itself.
However, the above-mentioned conventional cellophane/PE laminated film is quite inferior in the moisture-barrier property. A laminated film with a moistureproof cellophane exhibits a comparatively better moisture-barrier property. However, use of the laminated film with such moistureproof cellophane has a disadvantage that considerable labourious and costly expenses have to be paid for the moistureproof coating treatment of cellophane with vinylidene chloride, since many complicated process steps are required. In addition, due to the content of chlorine in the vinylidene chloride, use of the moistureproof cellophane is problematic in view of environmental protection.
As laminated films with outer layer made of other materials, such as polyester, polyamide (Ny) and polypropylene (PP), there may be enumerated those of the combinations of biaxially stretched polyester/PP, biaxially stretched Ny/PE and biaxially stretched PP/PP.
While these laminated films with resin outer layer exhibit better moisture-barriering property, as compared with those cellophane-base laminated films mentioned above, they are as yet unsatisfactory in the moisture-barrier property and, in addition, reveal a high tearing strength of the outer layer and are thus considerably inferior in the easy hand-tearability as compared with that of cellophane-based ones.
Namely, the moisture-barrier property is insufficient for, for example, a laminated film based on polyester in which the outer layer is constituted of a polyester resin and, moreover, the easy hand-tearability of such a laminated film is quite inferior due to the excessively high tearing strength of polyester resin.
Thus, it has conventionally been practised for increasing the easy tearability of packages of laminated film based on polyester to employ a technique, in which a hand tearing easiness is given by providing the package rim with a cut-in notch or in which the tearing strength of the packaging film is lowered by providing the outer layer of the film with many microperforations. However, such a technique complicates the production of the laminated film together with additional works.
Therefore, in order to increase the moisture-barrier property, it has been practiced to employ a technical measure of surface coating with vinylidene chloride, as in the moistureproof cellophane, or a vacuum deposition treatment with, for example, aluminum or silica to form a thin coating layer. For laminated films for packaging articles to be kept from moisture, there have been practiced to use those of combinations of, for example, vinylidene chloride-coated biaxially stretched polyester/PP, vinylidene chloride-coated biaxially stretched PP/PP, biaxially stretched PP/aluminum foil/PE, biaxially stretched PP/aluminum vacuum deposited-polyester/PE and biaxially stretched polyester/aluminum foil/PE.
However, employment of these technical measures of the vinylidene chloride treatment, vacuum deposition of aluminum and interposition of aluminum foil is bound inevitably with a complexity of production of the laminated film and an increase in the production cost.
As described above, no satisfactory laminated film has been proposed in the prior art, in which better resistances to heat and shock are realized together with attainment of superior moisture-barriering property and easy hand-tearability and, in addition, a simple and economical production is permitted.
In Japanese Patent Kokai Hei 4-276253 A, packaging vessels for pharmaceutical or food products having at least one layer of a thermoplastic polymer based on saturated norbornene, in particular, those of multilayer structure with laminated layer of other thermoplastic resin and having a layer of a polymer or its hydrogenation product of ring-opening polymerization of a monomer based on norbornene is disclosed. Here, however, the packaging vessels are assigned for medical uses, such as test tubes for fluid therapy, medical cells, blood bags, medical bottles and testing cells, or for general package of food products and there is no suggestion as to the packaging material composed of a hand-tearable thin film nor any mention as to the easy hand-tearability.